Pollucite-based ceramic with low CTE

ABSTRACT

A ceramic structure which is pollucite-based and has high refractoriness and high resistance to thermal shock. The inventive structure is suitable in high temperature applications such as a filtering particulates from diesel engine exhaust.

[0001] This application claims the benefit of U.S. ProvisionalApplication No. 60/246,661, filed Nov. 7, 2000, entitled“Pollucite-Based Ceramic with Low CTE”, by Morena et al.

BACKGROUND OF THE INVENTION

[0002] This invention relates to a pollucite-based, highly refractoryceramic having high resistance to thermal shock and being suitable forhigh temperature applications such as filters for filtering particulatesfrom diesel engines exhaust streams.

[0003] Pollucite which has the composition Cs₂O.Al₂O₃.4SiO₂ or CAS₄ isthe most refractory silicate known exhibiting a melting point above1900° C. However, a drawback associated with this material is its highcoefficient of thermal expansion at temperatures up to 400° C., which isusually about 120×10⁻⁷/° C., rendering the material a low resistance tothermal shock. Therefore, despite its high refractoriness, pollucitewould not be suitable for filtering applications of gas engines exhaustssuch as diesel particulate filters.

[0004] It would be considered an advancement in the art to obtain amaterial which has the high refractoriness of pollucite and also highresistance to thermal shock.

[0005] The present invention provides such a pollucite-based materialand a method of fabricating the same.

SUMMARY OF THE INVENTION

[0006] In accordance with one aspect of the invention, there is providedpollucite-based ceramic structures having high refractoriness and a highresistance to thermal shock, properties which make the inventivestructure extremely desirable in filtering applications of exhauststreams, in particular as filters for diesel exhaust engines.

[0007] In an embodiment the inventive ceramic structures comprises afirst phase having a stoichiometry of Cs₂O.Al₂O₃.4SiO₂ (CAS₄) and asecond phase having a stoichiometry Cs₂O.Al₂O₃.2SiO₂ (CAS₂).

[0008] In another embodiment the inventive ceramic structures furtherinclude a third phase selected from the group consisting ofSrO.Al₂O₃.2SiO₂ (SAS₂), SrO.SiO₂ (SrSiO₃) and combinations thereof.

[0009] The inventive structures have high thermal expansion anisotropyof between 1400-1450 ppm, as calculated from dimensional change ΔL/L₀over a temperature range from room temperature to 1000° C. and anaverage coefficient of thermal expansion from room temperature to 1000°C. of about −10×10⁻⁷/° C. to +25×10⁻⁷/° C., preferably −5×10⁻⁷/° C. to15×10⁻⁷/° C. For the two phase embodiment a CAS₄-CAS₂ I-ratio, definedas the ratio the intensity of the major peak of the CAS₄ phase atapproximately 3.42 Å to the intensity of the major peak of the CAS₂ at3.24 Å is about 0.25 to 3.0, preferably about 0.5 to 2, and mostpreferably about 1.0.

[0010] An advantage of the inventive structure is its suitability inhigh temperature applications such as filtering particulates from dieselengine exhaust. In particular the inventive structure is especiallysuitable as a honeycomb diesel particular filter having an inlet end andan outlet end and a multiplicity of cells extending from the inlet endto the outlet end, the cells having porous walls, wherein part of thetotal number of cells at the inlet end are plugged along a portion oftheir lengths, and the remaining part of cells that are open at theinlet end are plugged at the outlet end along a portion of theirlengths, so that an engine exhaust stream passing through the cells ofthe honeycomb from the inlet end to the outlet end flows into the opencells, through the cell walls, and out of the structure through the opencells at the outlet end. Diesel particulate filters having the inventivestructure have been obtained.

[0011] In accordance with another aspect of the invention, there isprovided a method of producing a formable mixture that involvescombining a dry blend material consisting essentially of 70-90%, byweight, of a glass frit and 10-30%, by weight, Al₂O₃, a solvent selectedfrom the group consisting of deionized water, an emulsion consistsessentially of, about 95%, by weight, deionized water, about 0.7%, byweight, triethanolamine and about 4.3%, by weight, oleic acid, andcombinations thereof, and a polymer selected from the group consistingof a crosslinked polyacrylic acid copolymer, a polyethylene oxidepolymer, and combinations thereof.

[0012] Up to 30%, by weight, SrO may be substituted for Cs₂O, theresulting ceramic structure then including a third phase selected fromthe group consisting of SrO.Al₂O₃.2SiO₂ (SAS₂), SrO.SiO₂ (SrSiO₃) andcombinations thereof. In forming the mixture containing the glass frithaving SrO substituted for cesium, a suitable polymer is anaqueous-based cellulose ether polymer selected from the group consistingof methylcellulose or hydroxylpropyl methylcellulose. Unlike the Cs₂Oglass frit, the Cs₂O—SrO glass frit has increased aqueous stabilitybecause strontium prevents the cesium from leaching and reacting withwater to form a gel or cementitious mixture which is incapable of beingshaped.

[0013] In accordance with another aspect of the invention, the formablemixture is shaped by extrusion to form a green monolithic structure,such as a honeycomb, which is then fired in an electric furnace at atemperature of about 1350 to 1550° C. over a period of about 6 to 12hours, and held at a maximum temperature for about 4 to 12 hours to formthe final product structure.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 illustrates a comparison between the thermal expansion ofExample 1, a pure pollucite structure, and Example 4, the diphasicinventive structure.

[0015]FIG. 2 illustrates a comparison between the coefficient of thermalexpansion Example 1, a pure pollucite structure, and Example 4, a twophase structure.

[0016]FIG. 3 illustrates the axial expansion curves for the CAS₂ phaseas determined by high temperature x-ray diffraction.

[0017]FIG. 4 shows a micrograph depicting the microstructure of theinventive material.

DETAILED DESCRIPTION OF THE INVENTION

[0018] The invention relates to pollucite-based ceramic structures whichhave high refractoriness and high resistance to thermal shock.

[0019] In one embodiment, the inventive ceramic structures comprise apollucite phase, Cs₂O.Al₂O₃.4SiO₂ or CAS₄ and a cesium aluminumdi-silicate phase, Cs₂O.Al₂O₃.2SiO₂ or CAS₂.

[0020] In another embodiment the inventive ceramic structures furtherinclude another phase selected from the group consisting ofSrO.Al₂O₃.2SiO₂ (SAS₂), SrO.SiO₂ (SrSiO₃) and combinations thereof.

[0021] The inventive structures are highly refractory with a meltingpoint in excess of 1800° C., and highly resistant to thermal shock withan average coefficient of thermal expansion from room temperature to1000° C. of −10×10⁻⁷/° C. to +25×10⁻⁷/° C., preferably −5×10⁻⁷/° C. to15×10⁻⁷/° C., due to a high thermal expansion anisotropy (i.e., widelydiffering expansions among different crystallographic axes). Theaforementioned advantageous properties make the inventive structuresextremely suitable for use in harsh environments and high temperatureapplications.

[0022] Specifically, the inventive structure are suitable in filtrationapplications for the removal of particulate material (i.e., carbon soot)from diesel engine exhaust streams, such as diesel particulate filters,in which regeneration of the filter by burning of the carbon soot canresult in locally high temperatures within the filter.

[0023] While the filter can have any shape or geometry suitable for aparticular application, it is preferred that it be multicellularstructure such as a honeycomb structure. Honeycombs are multicellularbodies having an inlet and outlet end or face, and a multiplicity ofcells extending from inlet end to outlet end, the cells having porouswalls.

[0024] Generally honeycomb cell densities range from about 4 cells/cm²(25 cells/in²) to about 93 cells/cm² (600 cells/in²). Examples ofhoneycombs produced by the process of the present invention, although itis to be understood that the invention is not limited to such, are thosehaving about 16 cells/cm² (100 cells/in²) to about 31 cells/cm² (200cells/in²), with a wall thickness of about 0.3 to 0.4 mm (10 to 15mils). Typically wall thicknesses are from about 0.07 to about 0.8 mm(about 3 to 30 mils).

[0025] It is preferred to have part of the total number of cells pluggedto allow better flow through the porous walls. A portion of the cells atthe inlet end or face are plugged with a paste having same or similarcomposition to that of the green body, as described in U.S. Pat. No.4,329,162 which is herein incorporated by reference. The plugging isonly at the ends of the cells which is typically to a depth of about 5to 20 mm, although this can vary. A portion of the cells on the outletend but not corresponding to those on the inlet end are plugged.Therefore, each cell is plugged only at one end. The preferredarrangement is to have every other cell on a given face plugged as in acheckered pattern.

[0026] This plugging configuration allows for more intimate contactbetween the exhaust stream and the porous wall of the substrate. Theexhaust stream flows into the substrate through the open cells at theinlet end, then through the porous cell walls, and out of the structurethrough the open cells at the outlet end. Filters of the type hereindescribed are known as a “wall flow” filters since the flow pathsresulting from alternate channel plugging require the exhaust beingtreated to flow through the porous ceramic cell walls prior to exitingthe filter.

[0027] The invention also relates to a method for fabricating theinventive structures by forming a mixture comprising a dry blend, asolvent, a polymer and optionally a pore former. The mixture may furtherbe shaped into a green body, such as by extrusion into honeycombstructures, and then fired to form the final product structure.

[0028] The dry blend comprises a precursor glass frit and alumina. Theprecursor glass frit is prepared by melting SiO₂-, Cs₂O-, andSrO₂-containing materials at temperatures no higher than 1650° C. toobtain a glass frit having a SiO₂/Cs₂O or SiO₂/(Cs₂O+SrO) molar ratio ofbetween 2.0-4.0, preferably 2.0-3.0 and most preferably 2.5.

[0029] Cesium-silicate glasses with cesium oxide greater than about 50weight percent (wt. %) on an oxide basis tend to be hygroscopic atambient temperatures. To alleviate this condition a minor amount ofalumina (Al₂O₃) of about 3-5 wt. % on an oxide basis may be added tosignificantly improve the moisture stability. Greater levels than 5 wt.% may increase the viscosity of the glass frit.

[0030] For the two-phase embodiment the composition of the precursorglass frit consists essentially, expressed in weight percent on an oxidebasis, of 60-68% Cs₂O, 29-35% SiO₂, and optionally 3-5% Al₂O₃, thelatter being added to improve the moisture stability of the glass. Thepreferred composition consists essentially, expressed in weight percenton an oxide basis, of 62% Cs₂O, 33% SiO₂, and optionally 5% Al₂O₃.

[0031] For the three-phase embodiment up to 30%, by weight, SrO issubstituted for Cs₂O, to obtain a third phase selected from the groupconsisting of SrO.Al₂O₃.2SiO₂ (SAS₂), SrO.SiO₂ (SrSiO₃) and combinationsthereof. It has been found that SrO substitution for Cs₂O dramaticallyimproves the aqueous stability of the glass frit in the presence of anaqueous polymer, even in the absence of alumina. This is becausestrontium prevents the cesium in the glass frit from leaching andreacting with water. This reaction may hinder the formability of themixture, by producing a gel or cementitious consistency in the mixture.

[0032] In a preferred embodiment the composition of the dry blendconsists essentially, expressed in percent by weight based on the totalweight of powder materials, of about 70-90% glass frit and 10-30%alumina, most preferably 85% glass frit and 15% alumina.

[0033] The solvent is selected from the group consisting of deionizedwater, an emulsion consisting essentially of about 95% deionized water,about 0.7% triethanolamine (such as TEA 99 manufactured by DOW ChemicalCo. Midland, Mich.) and about 4.3% oleic acid, by weight based on 100grams of emulsion, and combinations thereof. Isopropanol may also besuitable.

[0034] The polymer needs to be tolerant of the alkaline environmentcreated by the glass frit and not be detrimentally affected by leachatesfrom the glass frit. The polymer aids in the mixing, prevents watermigration, creates a plastic yield stress in the batch, and improvesgreen strength.

[0035] A suitable polymer is selected from the group consisting of acrosslinked polyacrylic acid copolymer, such as Carbopol 2020(manufactured by B F Goodrich, Cleveland Ohio), a polyethylene oxidepolymer, such as Polyox WSR Coagulant (manufactured by Union Carbide,Danburry Conn.), and combinations thereof. Preferably, the crosslinkedpolyacrylic acid copolymer is preferred for the mixture containing theCs₂O glass frit.

[0036] Another suitable polymer is an aqueous-based cellulose etherpolymer, preferably selected from the group consisting ofmethylcellulose and hydroxylpropyl methylcellulose. The aqueous-basedcellulose ether polymer is preferred for the mixture containing theCs₂O—SrO glass frit, because as described herein above the strontiumdramatically improves the aqueous stability of the glass frit in thepresence of water.

[0037] The mixture may also contain a pore former, as an optionalingredient. A pore former is a fugitive particulate material whichevaporates or undergoes pyrolysis during drying or heating of the greenbody to obtain a desired, usually larger porosity and/or coarser medianpore diameter than would be obtained otherwise. When a pore former isused, it is advantageous that it be a particulate pore former. Apreferred pore former is graphite in an amount of about 0-25%, having amedian particle size of at least 10 micrometers and more preferably ofat least 25 micrometers.

[0038] Preferably the mixture comprises about 50-85%, by weight, dryblend, about 15-30%, by weight, solvent, about 0.1-8%, by weight,preferably about 0.1-4%, by weight, polymer, and about 0-25%, by weight,graphite.

[0039] After blending, the mixture may be shaped into a green body byany suitable means, preferably the mixture is shaped into a honeycombstructure by extrusion. The extrusion operation can be done using ahydraulic ram extrusion press, or a two stage de-airing single augerextruder, or a twin screw mixer with a die assembly attached to thedischarge end. In the latter, the proper screw elements are chosenaccording to material and other process conditions in order to build upsufficient pressure to force the extruded through the die. The extrusioncan be vertical or horizontal.

[0040] The resulting green honeycomb structure may be optionally dried.To obtain the final product structure the green honeycomb structure isheated to a maximum temperature of about 1350° C. to 1550° C. over aperiod of about 6 to 12 hours, and held at the maximum temperature forabout 4 to 12 hours. The firing may be conducted in an electricallyheated furnace.

EXAMPLES

[0041] To more fully illustrate the invention, the followingnon-limiting examples are presented.

[0042] Table I below reports compositions for the precursor glasses usedto make the glass frits for the dry blends reported in Table II. Thecomposition for each glass is expressed in terms of weight percent onthe oxide basis.

[0043] For glasses OAM and OAP, a portion of Cs₂O was substituted withLi₂O. In the present invention, up to 2 wt. % of Li₂O may be substitutedfor Cs₂O. It has been found that the lithia substitution is beneficialin the inventive bodies in that it reduces macro- or gross-cracking inthe final structure, however, it is to be understood that it isoptional. Glasses NZC, NZO, and OAM are used to form the two-phasestructure comprising a first phase Cs₂O.Al₂O₃.4SiO₂ (CAS₄) and a secondphase Cs₂O.Al₂O₃.2SiO₂ (CAS₂).

[0044] In glass OAT a portion of Cs₂O was substituted with SrO. GlassOAT is used to form the three-phase structure comprising a first phaseCs₂O.Al₂O₃.4SiO₂ (CAS₄), a second phase Cs₂O.Al₂O₃.2SiO₂ (CAS₂) and athird phase selected from the group consisting of SrO.Al₂O₃.2SiO₂(SAS₂), SrO.SiO₂ (SrSiO₃) and combinations thereof. In the presentinvention up to 30 wt. % of SrO may be substituted for Cs₂O in the glassfrit.

[0045] The precursor glass raw materials were ballmilled together toassist in obtaining a homogeneous melt, charged into platinum crucibles,and inserted into a furnace at 1650° C. After about 12 to 16 hours, themelts, being sufficiently fluid, were poured through water cooled steelrolls to form a thin ribbon of glass which was subsequently dryballmilled to particles having a mean size of about 5-40 microns. Thefrit may be calcined for several hours at 600° C. to eliminate anyadsorbed water from the surface of the particles.

[0046] To form the dry blend the precursor glasses were mixed withpowdered α-alumina having a mean particle size of about 0.6 microns, inamounts as reported in Table 1. Comparative example 1 contains calcinedclay instead of alumina. In inventive example 3 precursor Glass OAM hada mean particle size of about 5-10 microns, and in inventive example 4precursor Glass OAM had a mean particle size of about 10-20 microns.

[0047] Table II reports the compositions and firing conditions used inthe preparation of the inventive structures. All parts, portions, andpercentages are based on the total weight of the raw materials, unlessotherwise stated. The weight percents of the solvent and polymer arecalculated as superadditions with respect to the raw material powdersolids by the following formula: [(weight of solvent, polymer or otheradditive)÷(weight units of powder material)]×100.

[0048] Asbury 4740 graphite (manufactured by Asbury Carbons, Inc.,Asbury, N.J.) was added as a pore former to some of the examples.

[0049] All the dry raw materials were weighed into a container and drymixed to provide homogenization. The mixtures were then transferred intoa stainless steel muller to which the liquid component was graduallyadded in a quantity sufficient to impart plasticity to the mixture.

[0050] The plasticized mixture was then extruded into a cellularstructures having about 200 cells per square inch (31 cells/cm²), a wallthickness of about 0.010 to 0.015 inches (0.025 to 0.038 cm), and adiameter of about 2.54 cm (1 in) or about 5.08 cm (2 in).

[0051] The cellular samples were optionally dried and then fired in anelectrically heated furnace at a rate of about 300-400° C./hr to amaximum temperature of 1350° C. to 1550° C., held for 4-12 hours, andcooled by shutting off power to the furnace.

[0052] The samples were then evaluated for phase assemblage as reportedin table II. Phase assemblage was identified by powder x-raydiffraction. Comparative example 1 is pure pollucite CAS₄ with a majorpeak at 3.42 Å. It used as a comparative example. Inventive example 2,3, 4, and 5 comprise the phases Cs₂O.Al₂O₃.4SiO₂ (CAS₄), andCs₂O.Al₂O₃.2SiO₂ (CAS₂). Inventive example 6 includes the phasesCs₂O.Al₂O₃.4SiO₂ (CAS₄), Cs₂O.Al₂O₃.2SiO₂ (CAS₂) and SrO.Al₂O₃.2SiO₂(SAS₂).

[0053] Table II further reports the intensity of the major CAS₄ peak at3.42 Å to the major CAS₂ peak at 3.24 Å for the inventive structurescomprising phases Cs₂O.Al₂O₃.4SiO₂ (CAS₄), and Cs₂O.Al₂O₃.2SiO₂ (CAS₂).This ratio is used herein as the “CAS₄-CAS₂ I-ratio”. Although thepercent peak heights are not actually equivalent to the weightpercentages or volume percentages of the phases in the fired bodies,they do provide a qualitative comparison of the amounts of these phasesamong the examples. It has been observed that CAS₄-CAS₂ I-ratios ofabout 0.25 to 3.0, preferably about 0.5 to 2, and most preferably 1.0,are beneficial in the inventive two-phase structures. At values lowerthan 0.25 the two-phase structure tends to be weak, and to fracturespontaneously due to what is believed to be gross microcracking in themicrostructure.

[0054] Also reported in table II are the mean coefficient of thermalexpansion (CTE) over the temperature range from room temperature to1000° C. as measured using a dilatometer. For the inventive examples theCTE ranges between −3.0×10⁻⁷/° C. to +12×10⁻⁷/° C., considerably lowerthan 38×10⁻⁷/° C. for the comparative example 1.

[0055] Referring now to FIG. 1 therein illustrated is a comparisonbetween the thermal expansion behavior of comparative example 1 (purepollucite) and inventive example 4 (structure with phasesCs₂O.Al₂O₃.4SiO₂ (CAS₄), and Cs₂O.Al₂O₃.2SiO₂ (CAS₂)) as determined bydilatrometry. The dimensional change ΔL/L₀ in parts per million (ppm) isplotted as a function of temperature from room temperature to 1000° C.for both heating and cooling cycles. In comparative example 1, theheating curve nearly overlaps the cooling curve, and there is negligiblehysteresis detected. The maximum thermal expansion anisotropy is thelargest difference between the heating and cooling curves at a giventemperature in the range from room temperature to 1000° C. For inventiveexample 4, the thermal expansion anisotropy is calculated to be about1435 ppm.

[0056] In inventive example 4 the behavior of the heating curve is muchdifferent than the behavior of the cooling curve; more specifically thethermal expansion of inventive example 4 is characterized by extremehysteresis or high thermal expansion anisotropy. Extreme hysteresis orhigh thermal expansion anisotropy is indicative of differingmicrostructures present ceramic between heating and cooling, as a resultof the generation of microcracks during cooling and their partialhealing during heating. The presence of the microcracks creates a freeinternal volume manifested by a low or negative mean coefficient ofthermal expansion, as can be observed for inventive example 4 in FIG. 2.

[0057] Therefore, an advantage of the inventive structures is a highthermal expansion anisotropy of between 1400-1450 ppm, as calculatedfrom the dimensional change ΔL/L₀ over a temperature range from roomtemperature to 1000° C.

[0058] In FIG. 2, the CTE of comparative example 1 reveals a highexpansion of about 60-100×10⁻⁷/° C. over a temperature range from roomtemperature to about 200° C. The CTE does decrease to about 40×10⁻⁷/° C.at 1000° C., however it is still unacceptably high for dieselparticulate filters.

[0059] Due to a high thermal expansion anisotropy, the CTE curve of thetwo phase material of inventive example 4 differs appreciably from thepure pollucite material of comparative example 1. The initial portion ofthe heating curve, from room temperature to about 200° C. reflects thedominance of the CAS₄ phase and the relatively low percentage ofmicrocracks, with a value of CTE at about 20×10⁻⁷/° C.; however, the CTEis still considerably lower than that of comparative example 1 for thesame temperature range. From about 350° C. to 1000° C., the CTEdecreases with increasing temperature to between zero to −10×10⁻⁷/° C.,as a result of microcracking. During cooling, a negative CTE ismaintained which results in excellent resistance to thermal shock forthe inventive body.

[0060] Although not intended to be bound by theory it is believed thatthe hysteresis behavior is a result of microcracking with increasingtemperature. In turn the microcracking is a function of extreme thermalexpansion anisotropy (widely differing expansions among differentcrystallographic axes) within the CAS₂ lattice and the large CTEdifferences between the two phases. The CAS₂ phase is orthorhombic withwidely differing expansions among its crystallographic axes, as shown inFIG. 3 which illustrates the axial thermal expansion data for thea-axis, b-axis, and c-axis as obtained by hot stage x-ray diffraction.Calculation of the CTE over a temperature range from room temperature to800° C., for each axis of the CAS₂ phase reveals the following highdegree of anisotropy: a-axis=+500×10⁻⁷/° C.; b-axis=−130×10⁻⁷/° C.;c-axis=+30×10⁻⁷/° C. By contrast pollucite is a cubic crystal, and thusdisplays no thermal expansion anisotropy among the crystallographicaxes.

[0061]FIG. 4 is a Scanning Electron Microscope micrograph showing themicrostructure of a two phase CAS₂-CAS₄ material of the instantinvention at a magnification of 4000×. Two distinct phases can beobserved: a highly microcracked matrix which is identified as theCAS₂-phase and large, isolated uncracked regions which is identified asthe CAS₄-phase. The CAS₂-phase is small approximately 1-2 microns insize compared to the CAS₄-phase of about 10-20 microns. As can beobserved the CAS₄-phase enclose and isolate the CAS₂-phase highlymicrocracked regions. It has been found that this microstructure enablesthe inventive structure to posses a combination of negative to near-zeroCTE, extreme thermal expansion anisotropy and high refractoriness. TABLEI Glass NZC NZO OAM OAT Dry Blend Raw Materials Cs₂O 67.7 62.3 60.0 43.3SrO — — — 15.9 Li₂O — — 0.6 — SiO₂ 28.9 32.9 34.5 36.9 Al₂O₃ 3.1 4.5 4.73.9 SiO₂/Cs₂O molar ratio 2.0 2.5 2.7 — SiO₂/(Cs₂O + SrO) — — — 2.0molar ratio Dry Blend Composition Example 1 (comp.) 70 wt. % NZC + 30wt. % clay Example 2 (inv.) 85 wt. % NZO + 15 wt. % alumina Example 3(inv.) 85 wt. % OAM + 15 wt. % alumina Example 4 (inv.) 85 wt. % OAM +15 wt. % alumina Example 5 (inv.) 85 wt. % OAM + 15 wt. % aluminaExample 6 (inv.) 80 wt. % OAM + 20 wt. % alumina

[0062] TABLE II Example Number 1 2 3 4 5 6 Example Type Comp. Inv. Inv.Inv. Inv. Inv. Mixture Components Dry Blend 63.5 58.0 62.2 63.5 63.564.3 Liquid System Polymer Carbopol 2020 ® 2.7 2.5 2.8 2.8 2.3 — PolyvoxWSR ® — — — — 0.46 — Methylcellulose (Methocel — — — — — 2.8 F40M ®)Solvent Deionized Water — 8.7 — — — — Emulsion D 21.0 19.2 19.4 21.019.1 20.1 Isopropanol — — — — 1.91 — Pore Forming Agent Graphite 12.711.6 15.6 12.7 12.7 12.9 Firing Conditions Furnace Type electricelectric electric electric electric electric Maximum Temperature (° C.)1350 1550 1350 1350 1350 1400 Hold Time (hours) 4 12 4 4 4 6 PhaseAssemblage Cs₂O.Al₂O₃.4SiO₂ (CAS₄) x x x x x x Cs₂O.Al₂O₃.2SiO₂ (CAS₂) —x x x x x SrO.Al₂O₃.2SiO₂ (SAS₂) — — — — — x CAS₄—CAS₂ I-Ratio 14.0 0.630.8 2.0 — — Properties of Fired Ware Mean CTE from 22 to 1000° C. 38.012.0 1.0 −3.0 — 4.7 (10⁻⁷/° C.)

What is claimed is:
 1. A ceramic structure comprising a first phaseCs₂O.Al₂O₃.4SiO₂ (CAS₄) and a second phase Cs₂O.Al₂O₃.2SiO₂ (CAS₂), andhaving high thermal expansion anisotropy of between 1400-1450 ppm, ascalculated from dimensional change ΔL/L₀ over a temperature range fromroom temperature to 1000° C. and an average coefficient of thermalexpansion from room temperature to 1000° C. of about −10×10⁻⁷/° C. to+25×10⁻⁷/° C.
 2. The ceramic structure of claim 1 wherein the averagecoefficient of thermal expansion from room temperature to 1000° C. isabout −5×10⁻⁷/° C. to +15×10⁻⁷/° C.
 3. The ceramic structure of claim 1further having a CAS₄-CAS₂ I-ratio, defined as the ratio the intensityof the major peak of the CAS₄ phase at approximately 3.42 Å to theintensity of the major peak of the CAS₂ at 3.24 Å, of about 0.25 to 3.0.4. The structure of claim 1 where in the CAS₄-CAS₂ I-ratio is 0.5 to2.0.
 5. The structure of claim 3 wherein the CAS₄-CAS₂ I-ratio is 1.0.6. The structure of claim 2 further including a third phase selectedfrom the group consisting of SrO.Al₂O₃.2SiO₂ (SAS₂), SrO.SiO₂ (SrSiO₃)and combinations thereof.
 7. A diesel particulate filter comprising adiphasic highly refractory ceramic having a first phase Cs₂O.Al₂O₃.4SiO₂(CAS₄) and a second phase Cs₂O.Al₂O₃.2SiO₂ (CAS₂) wherein the ceramichas high thermal expansion anisotropy from room temperature to 1000° C.,an average coefficient of thermal expansion from room temperature to1000° C. of about −10×10⁻⁷/° C. to +25×10⁻⁷/° C., and a CAS₄-CAS₂I-ratio, defined as the ratio the intensity of the major peak of theCAS₄ phase at approximately 3.42 Å to the intensity of the major peak ofthe CAS₂ at 3.24 Å, of about 0.25 to 3.0, wherein the diesel particulatefilter comprises a honeycomb body, the honeycomb having an inlet end andan outlet end and a multiplicity of cells extending from the inlet endto the outlet end, the cells having porous walls, wherein part of thetotal number of cells at the inlet end are plugged along a portion oftheir lengths, and the remaining part of cells that are open at theinlet end are plugged at the outlet end along a portion of theirlengths, so that an engine exhaust stream passing through the cells ofthe honeycomb from the inlet end to the outlet end flows into the opencells, through the cell walls, and out of the structure through the opencells at the outlet end.
 8. The diesel particulate filter of claim 7wherein the CAS₄-CAS₂ I-ratio is 0.5 to 2.0.
 9. The diesel particulatefilter of claim 8 wherein the CAS₄-CAS₂ I-ratio is 1.0.
 10. The dieselparticulate filter of claim 7 wherein the average coefficient of thermalexpansion from room temperature to 1000° C. is about −5×10⁻⁷/° C. to+15×10⁻⁷/° C.
 11. A diesel particulate filter comprising a highlyrefractory ceramic having a first phase Cs₂O.Al₂O₃.4SiO₂ (CAS₄), asecond phase Cs₂O.Al₂O₃.2SiO₂ (CAS₂), and a third phase selected fromthe group consisting of SrO.Al₂O₃.2SiO₂ (SAS₂), SrO.SiO₂ (SrSiO₃) andcombinations thereof, wherein the ceramic has high thermal expansionanisotropy from room temperature to 1000° C. and an average coefficientof thermal expansion from room temperature to 1000° C. of about−10×10⁻⁷/° C. to +25×10⁻⁷/° C., wherein the diesel particulate filtercomprises a honeycomb body, the honeycomb having an inlet end and anoutlet end and a multiplicity of cells extending from the inlet end tothe outlet end, the cells having porous walls, wherein part of the totalnumber of cells at the inlet end are plugged along a portion of theirlengths, and the remaining part of cells that are open at the inlet endare plugged at the outlet end along a portion of their lengths, so thatan engine exhaust stream passing through the cells of the honeycomb fromthe inlet end to the outlet end flows into the open cells, through thecell walls, and out of the structure through the open cells at theoutlet end.
 12. The diesel particulate filter of claim 11 wherein theaverage coefficient of thermal expansion from room temperature to 1000°C. is about −5×10⁻⁷/° C. to +15×10⁻⁷/° C.
 13. A method of producing aformable mixture, the method comprising combining a dry blend materialconsisting essentially of 70-90%, by weight, of a glass frit and 10-30%,by weight, Al₂O₃, a solvent selected from the group consisting ofdeionized water, an emulsion consists essentially of, about 95%, byweight, deionized water, about 0.7%, by weight, triethanolamine andabout 4.3%, by weight, oleic acid, and combinations thereof, and apolymer selected from the group consisting of a crosslinked polyacrylicacid copolymer, a polyethylene oxide polymer, and combinations thereof.14. The method of claim 13 wherein the polymer is a crosslinkedpolyacrylic acid copolymer.
 15. The method of claim 13 wherein the glassfrit consists essentially, expressed in weight percent on an oxidebasis, of 60-68% Cs₂O, 29-35% SiO₂, and optionally 3-5% Al₂O₃.
 16. Themethod of claim 13 wherein up to 2%, by weight, Li₂O is substituted forCs₂O.
 17. The method of claim 13 wherein up to 30%, by weight, SrO, issubstituted for Cs₂O.
 18. The method of claim 17 wherein the polymer isan aqueous-based cellulose ether polymer.
 19. The method of claim 18wherein the aqueous-based cellulose ether polymer is selected from thegroup consisting of methylcellulose or hydroxylpropyl methylcellulose.20. The method of claim 19 wherein the solvent is an emulsion consistsessentially of, about 95%, by weight, deionized water, about 0.7%, byweight, triethanolamine and about 4.3%, by weight, oleic acid, and thepolymer is methylcellulose.
 21. The method of claim 13 comprising theadditional step of shaping the mixture into a monolithic structure. 22.The method of claim 21 wherein the mixture is shaped by extrusion. 23.The method of claim 22 wherein the mixture is extruded into a honeycomb.24. A method of making a monolithic structure for high temperaturefiltration applications, the method comprising: a) forming a mixturecomprising: i) about 50-85%, by weight, dry blend consisting essentiallyof: 1) 70-90%, by weight, of a glass frit consisting essentially,expressed in weight percent on an oxide basis, of 60-68% Cs₂O, 29-35%SiO₂, and optionally 3-5% Al₂O₃; and, 2) 10-30%, by weight, Al₂O₃; and,ii) 15-30%, by weight, of a solvent selected from the group consistingof deionized water, an emulsion consists essentially of, about 95%, byweight, deionized water, about 0.7%, by weight, triethanolamine andabout 4.3%, by weight, oleic acid, and combinations thereof; iii)0.1-8%, by weight, of a polymer selected from the group consisting of acrosslinked polyacrylic acid copolymer, a polyethylene oxide polymer,and combinations thereof; and, iv) 0-25%, by weight, of a pore former;b) shaping the mixture to form a green body; and, c) firing the greenbody in an electric furnace at a temperature of about 1350 to 1550° C.over a period of about 6 to 12 hours, and held at a maximum temperaturefor about 4 to 12 hours.
 25. The method of claim 24 wherein dry blendconsists essentially of about 85%, by weight, glass frit and about 15%,by weight, alumina.
 26. The method of claim 24 wherein up to 2%, byweight, Li₂O is substituted for Cs₂O.
 27. The method of claim 26 whereinthe polymer is added at 0.1-4%, by weight.
 28. The method of claim 27wherein the polymer is a crosslinked polyacrylic acid copolymer.
 29. Themethod of claim 24 wherein up to 30%, by weight, SrO, is substituted forCs₂O.
 30. The method of claim 28 wherein the polymer is an aqueous-basedcellulose ether polymer.
 31. The method of claim 30 wherein theaqueous-based cellulose ether polymer is selected from the groupconsisting of methylcellulose or hydroxylpropyl methylcellulose.
 32. Themethod of claim 31 wherein the solvent is an emulsion consistsessentially of, about 95%, by weight, deionized water, about 0.7%, byweight, triethanolamine and about 4.3%, by weight, oleic acid, and thepolymer is methylcellulose.
 33. The method of claim 24 wherein the poreformer is graphite.
 34. The method of claim 24 wherein the mixture isshaped by extrusion into a honeycomb structure having an inlet end andan outlet end and a multiplicity of cells extending from the inlet endto the outlet end, the cells having porous walls.
 35. The method ofclaim 34 wherein every other cell is plugged to form a wall-flow filter.